Smart grid and renewable integration

Electrical and Electronics Engineering (B.E./B.Tech) · Power systems · free study notes for exams, placements and interviews

In short

A smart grid is a conventional power grid overlaid with two-way communication, measurement, automation and control so that generation, network and load can be observed and coordinated in near real time. Renewable integration is the engineering problem the smart grid is largely built to solve: wind and solar are variable, uncertain, non-synchronous and often connected at distribution voltage, which breaks assumptions of a grid designed for dispatchable synchronous machines. India's installed non-fossil capacity has crossed roughly half of total capacity, with a stated target of 500 GW non-fossil by 2030, so integration is now core grid-operations work rather than a research subject. It is examined in GATE EE (renewable integration, grid codes, FACTS/storage interaction), in university electives named Smart Grid or Renewable Energy Systems, and tested in interviews at POWERGRID, Grid-India, state transcos and discoms, NTPC, Adani Green, Tata Power, ReNew, Hitachi Energy, Siemens and Schneider.

Explained

What makes a grid "smart" - the functional layers

Sensing layer: smart meters (AMI), feeder-level IEDs, and PMUs (phasor measurement units) giving GPS time-stamped voltage/current phasors at 25-50 samples per second; PMU data feeds WAMS (wide-area measurement systems).
Communication layer: fibre/OPGW on transmission, RF mesh, cellular and PLC on distribution; protocols IEC 61850 (substation automation and, via 61850-90-7, DER), IEC 60870-5-104 and DNP3 (SCADA telemetry), IEC 61968/61970 CIM (data models), DLMS/COSEM (Indian smart-meter standard, IS 15959).
Control and application layer: SCADA/EMS at load dispatch centres; ADMS (advanced distribution management system) with DMS, OMS and FLISR (fault location, isolation and service restoration); volt-VAr optimisation; forecasting engines; DERMS for aggregating distributed resources.
Market/consumer layer: time-of-day and dynamic tariffs, prepaid metering, net/gross metering, demand response, EV smart charging.
Self-healing: automated sectionalisers and reclosers reconfigure the feeder after a fault, cutting outage minutes (SAIDI/SAIFI) without a crew visit.
Key contrast: the legacy grid is one-way, radial, manually restored and load-following; the smart grid is two-way, meshed/reconfigurable, self-healing and partly load-shaping.

Why renewables are hard to integrate

Variability: output changes with irradiance and wind speed - solar follows a bell curve, wind is stochastic with large ramps.
Uncertainty: forecast error means reserves must be carried; day-ahead solar MAPE of a few per cent at a pooled regional level but 10-20% for a single plant.
Non-synchronous, inverter-based: no rotating mass, so no inherent inertia; system inertia H falls and RoCoF (rate of change of frequency) after a trip rises, shrinking the time available for governors and load shedding to act.
Low short-circuit contribution: an inverter feeds about 1.1-1.5 pu current against 5-6 pu for a synchronous generator, so fault levels drop, protection sensitivity falls and voltage becomes "weak" (low SCR = short-circuit ratio).
Location: best wind/solar resources (Rajasthan, Gujarat, Tamil Nadu, Karnataka, Andhra Pradesh) are far from demand centres, demanding green-energy-corridor transmission; rooftop PV sits at LV and reverses power flow.
Duck curve: mid-day solar suppresses net load, then a steep evening ramp as solar falls and household demand peaks - this is India's main flexibility challenge, met by hydro, pumped storage, gas, part-loaded coal and now BESS.
Must-run status: under Indian practice RE is treated as must-run and curtailment is allowed mainly for grid security; frequent commercial curtailment is a regulatory dispute area.

Grid-connection technology for solar and wind

Solar PV plant: array → string/central inverter (MPPT, DC-AC) → inverter duty transformer (e.g. 0.8/33 kV) → pooling substation → 220/400 kV ISTS. MPPT holds operation at the maximum power point as irradiance and temperature move.
Wind generator types: Type 1 squirrel-cage induction (fixed speed, needs capacitors), Type 2 wound rotor with external resistance, Type 3 DFIG (rotor-side converter, ±30% speed range, partial-rating converter), Type 4 full converter PMSG/synchronous (full decoupling, best grid-code compliance). Types 3 and 4 dominate modern Indian installations.
Grid-following (GFL) inverters need an existing voltage to lock their PLL onto - they behave as current sources and cannot run islanded; grid-forming (GFM) inverters impose voltage magnitude and angle and can provide synthetic/virtual inertia and black-start.
Fault ride-through (LVRT/HVRT): the plant must stay connected through a voltage dip following a defined voltage-time envelope and inject reactive current to support recovery, instead of tripping and worsening the event.
Reactive power and voltage control: inverters operate within a P-Q capability curve (typically ±0.95 power factor at full output, wider at part load), supplemented by STATCOMs, SVCs, capacitor banks and reactors at the pooling substation.
Frequency support: droop/primary frequency response (power reduces as frequency rises), de-loading to hold headroom, synthetic inertia from converter control or storage, and fast frequency response from BESS.
Islanding and anti-islanding: an inverter must detect loss of mains and disconnect, typically within 2 s, to protect line workers and prevent out-of-phase reclosing; methods are passive (under/over voltage and frequency, RoCoF, vector shift) and active (impedance or frequency-shift injection); NDZ = non-detection zone where load matches generation and detection is hardest.

Storage, flexibility and demand side

Battery energy storage (BESS): Li-ion (LFP and NMC) dominates; rated in MW (power) and MWh (energy), ratio expressed as C-rate or duration (a 2-hour system is 0.5C). Round-trip efficiency about 85-92% at AC terminals.
Services stacked by BESS: energy arbitrage/time shift, peak shaving, frequency regulation and FFR, reserve, ramp-rate smoothing of RE, black-start support, and deferral of network reinforcement.
Pumped storage hydro (PSH): India's largest practical bulk store; round-trip efficiency about 70-80%, long life, large energy capacity, site-limited; several GW under development.
Other flexibility: flexible coal operation (technical minimum of coal units reduced to 55% as policy practice), hydro reservoir shifting, green hydrogen electrolysers as flexible load, interconnection with neighbouring regions.
Demand response and demand-side management: peak-time rebates, direct load control of agricultural feeders and water heating, time-of-day tariffs, EV V1G (smart charging) and V2G (bidirectional).
Microgrid: a bounded group of DERs and loads that can run grid-connected or islanded via a PCC with a static switch; control hierarchy is primary (droop, fast, local), secondary (restores voltage/frequency to nominal), tertiary (economic dispatch/grid exchange). AC, DC and hybrid topologies exist; India has many rural DC solar microgrids and campus hybrid microgrids.

Indian regulatory and standards framework

Electricity Act 2003 is the parent law; CERC regulates interstate matters and SERCs intrastate; Grid-India (NLDC/RLDCs) and SLDCs operate the system; CEA frames technical standards.
IEGC - Indian Electricity Grid Code. CERC notified a consolidated IEGC 2023, which covers RE forecasting, scheduling and deviation, ancillary services, reserves (primary, secondary, tertiary) and connectivity conditions; clauses are revised periodically, so cite the current amendment rather than a remembered number.
CEA (Technical Standards for Connectivity to the Grid) Regulations, amended over the years, set the fault-ride-through, reactive capability and frequency-response obligations for generating stations including RE.
Deviation Settlement Mechanism (DSM) prices the difference between schedule and actual; F&S (forecasting and scheduling) regulations make RE generators responsible for forecast accuracy, with error bands.
RPO (renewable purchase obligation) and the carbon/renewable consumption trajectory now flow from the Energy Conservation (Amendment) Act 2022; RE certificates were restructured by CERC's 2022 REC regulations.
Market and access: ISTS charge waiver for RE commissioned within notified windows (being phased out on a graded schedule), General Network Access (GNA) regulations 2022 replacing earlier connectivity/LTA framework, power exchanges (IEX, PXIL, HPX) with Green Term-Ahead and Green Day-Ahead markets, and RTM (real-time market, 15-minute blocks).
Operating band: IEGC frequency band is 49.90-50.05 Hz for operational purposes against 50 Hz nominal; India moved to 15-minute scheduling/settlement blocks, with a transition to 5-minute metering/settlement underway - check the current notification before quoting.
Metering and DER: CEA (Installation and Operation of Meters) Regulations require smart meters with prepaid capability; MoP rules 2022 on consumer rights cover rooftop-solar net metering up to 500 kW (state rules vary - many allow net metering only up to 10 kW and gross/net billing above).
Equipment standards: IS/IEC 61727 and IEC 62116 (PV inverter grid interface and anti-islanding), IEEE 1547 (widely referenced DER interconnection), IS 16169, IEC 61850 (automation), IEC 62052/62053 and IS 15959 (metering), CEA safety regulations for earthing and protection.
Programmes: RDSS (Revamped Distribution Sector Scheme) funding ~25 crore prepaid smart meters and loss-reduction works; Green Energy Corridors I and II; PM Surya Ghar: Muft Bijli Yojana for rooftop solar; National Smart Grid Mission and the earlier 14 NSGM pilot projects; PM-KUSUM for solar agricultural pumps.

Protection, power quality and cyber security

Protection impacts: reduced and bidirectional fault current, blinding of relays, sympathetic tripping of healthy feeders, loss of fuse-recloser coordination, and failure of distance relays due to low SCR - remedies are directional and adaptive relays, communication-assisted schemes, and revised settings per DER penetration study.
Power quality: inverter switching harmonics (limits per IEEE 519 / IEC 61000 series), DC injection, voltage rise on feeders with reverse flow, flicker from wind turbulence, and resonance with capacitor banks.
New stability problems: sub-synchronous control interaction (SSCI) between DFIG converters and series-compensated lines, converter-driven oscillations in weak grids, and control instability when SCR is low.
Cyber security: attack surface spans meters, RTUs, substation LANs and control centres; practice follows CEA Cyber Security in Power Sector Guidelines 2021, CERT-In reporting, ISO 27001 and IEC 62443, with mandatory Chief Information Security Officers and testing of imported equipment.
Data and privacy: load profiles from AMI reveal occupancy; access control and anonymisation matter in discom analytics.

Key points

Smart grid - an electricity network with two-way communication, measurement and automated control over generation, network, storage and load.
AMI - smart metering infrastructure (meter, communication, head-end, MDM) giving interval data, remote connect/disconnect and prepaid billing.
PMU/WAMS - GPS-synchronised phasor measurement at 25-50 Hz enabling wide-area oscillation and angle monitoring, far faster than 2-10 s SCADA scans.
Inertia - stored kinetic energy of rotating machines; inverter-based RE contributes none inherently, so RoCoF rises and frequency nadirs deepen.
Short-circuit ratio (SCR) - ratio of grid fault MVA to plant rating; below about 3 the grid is "weak" and converter controls can become unstable.
LVRT - obligation to ride through a voltage dip along a defined voltage-time curve while supporting reactive current.
Grid-forming inverter - sets its own voltage magnitude and angle, can island and provide virtual inertia; grid-following needs an external reference.
Duck curve - net-load shape with a solar-depressed midday and steep evening ramp; drives storage and flexible-generation needs.
Curtailment - deliberate reduction of RE output; in India RE is must-run and curtailment is permitted mainly for grid security.
DSM and F&S - Indian mechanisms charging RE generators for deviation from their declared 15-minute schedules.
IEGC 2023 - consolidated CERC grid code covering connectivity, scheduling, reserves and ancillary services; frequency band 49.90-50.05 Hz.
Net metering vs gross metering - net settles only the exported-minus-imported energy; gross buys all generation at a feed-in tariff and bills consumption separately.
Anti-islanding - mandatory loss-of-mains detection and disconnection (typically within 2 s), with a non-detection zone when load matches DER output.
BESS rating - MW sets power service capability, MWh sets duration; round-trip efficiency about 85-92%, PSH about 70-80%.
Microgrid control - primary (droop) → secondary (restore nominal) → tertiary (economics), with seamless islanding at the PCC.
FLISR/self-healing - automated fault isolation and restoration that improves SAIFI/SAIDI without field crews.
Capacity factor - actual energy ÷ (rated power × hours); about 17-22% for Indian solar PV, 25-35% for modern wind, against 60-70% for coal.
Protection blinding - DER infeed reduces current seen by the upstream relay, delaying or preventing tripping.

Formulas and examples

Capacity factor CF = E / (P_rated × T) — E in kWh/MWh, P_rated in kW/MW, T in hours (8,760 per year)
Performance ratio PR = E_actual / (H × A × η_STC) ≈ final yield ÷ reference yield — dimensionless, typically 0.75-0.82 for Indian PV
PV temperature correction: P = P_STC × [1 + γ (T_cell − 25)] — γ ≈ −0.0035 to −0.0045 per °C; T_cell in °C
Wind power P = ½ ρ A v³ C_p — ρ air density kg/m³, A = πR² swept area m², v wind speed m/s, C_p power coefficient (Betz limit 0.593)
Stored inertial energy E = H × S — H in s (MW·s/MVA), S in MVA; system H_sys = Σ(H_i S_i) / ΣS_i
RoCoF: df/dt = (f₀ × ΔP) / (2 × H_sys × S_sys) — Hz/s, ΔP in MW (loss of generation)
Frequency droop: ΔP/P_rated = −(1/R) × (Δf/f₀); system stiffness ΔP = −K Δf, K in MW/Hz
Short-circuit ratio SCR = S_fault at PCC / P_rated of plant
BESS round-trip efficiency η_RT = E_discharged / E_charged; usable energy = rated MWh × DoD × SoH
Reactive capability Q = P × tan(cos⁻¹ pf); inverter S = √(P² + Q²) ≤ S_rated (kVA)
Feeder voltage rise from DER ΔV ≈ (P R + Q X) / V — R, X line resistance/reactance in Ω, V in V
Deviation charge (DSM) = |Scheduled − Actual| × applicable rate per the current CERC DSM regulation

Example 1 - Solar plant energy, CF and temperature derating. A 50 MW (AC) solar plant in Rajasthan generates 91.25 GWh in a year.

CF = 91,250 MWh / (50 MW × 8,760 h) = 91,250 / 438,000 = 0.2083 → 20.8%.
Equivalent full-load hours = 0.2083 × 8,760 = 1,825 h/year (i.e. 5.0 kWh per kW(AC) per day class output).
Module derating: if a module is 400 Wp at STC with γ = −0.004/°C and cell temperature reaches 60 °C, P = 400 × [1 + (−0.004)(60 − 25)] = 400 × (1 − 0.14) = 344 W - a 14% loss from temperature alone, which is why Indian plants are often DC-oversized (DC:AC ≈ 1.3).

Example 2 - Wind turbine output. A 2 MW turbine, rotor diameter 100 m, ρ = 1.225 kg/m³, C_p = 0.42, wind 12 m/s.

A = π × 50² = 7,853.98 m².
v³ = 12³ = 1,728.
P = 0.5 × 1.225 × 7,853.98 × 1,728 × 0.42 = 0.5 × 1.225 = 0.6125; 0.6125 × 7,853.98 = 4,810.56; × 1,728 = 8,312,648 W; × 0.42 = 3.49 MW of aerodynamic power - above the 2 MW rating, so the turbine is already pitch-limited at 12 m/s and delivers its rated 2 MW.
At 8 m/s: v³ = 512 → 4,810.56 × 512 = 2,463,007 W; × 0.42 = 1.03 MW. Halving the wind from 12 to 6 m/s would cut aerodynamic power by a factor of 8 - the cube law is the key insight.

Example 3 - Inertia and RoCoF as RE share rises. A 10,000 MVA system; initially 80% of capacity is synchronous with H = 4 s, 20% inverter-based with H = 0.

H_sys = (0.8 × 10,000 × 4) / 10,000 = 3.2 s. Loss of a 500 MW unit, f₀ = 50 Hz: df/dt = (50 × 500) / (2 × 3.2 × 10,000) = 25,000 / 64,000 = 0.391 Hz/s.
Now RE share rises to 60% synchronous / 40% inverter: H_sys = 0.6 × 4 = 2.4 s → df/dt = 25,000 / 48,000 = 0.521 Hz/s, a 33% faster fall.
If a BESS supplies 150 MW of synthetic inertia/fast response instantly, effective ΔP = 350 MW → df/dt = (50 × 350)/48,000 = 0.365 Hz/s, restoring the original margin.

Example 4 - Droop response and system stiffness. A 200 MW generator on 5% droop, f₀ = 50 Hz, frequency falls to 49.8 Hz.

ΔP = −(1/0.05) × (−0.2/50) × 200 = 20 × 0.004 × 200 = 16 MW increase.
If the whole system has stiffness K = 4,000 MW/Hz, a 500 MW loss settles at Δf = −500/4,000 = −0.125 Hz, i.e. 49.875 Hz - just below the IEGC 49.90-50.05 Hz band, so secondary control (AGC) must restore it.

Example 5 - BESS sizing for an evening ramp, with cost. A discom must shift 40 MWh of solar energy into a 4-hour evening peak and needs 10 MW of discharge power.

Power rating 10 MW, energy at AC terminals 40 MWh → 4-hour duration (0.25C).
With η_RT = 88%, charging energy needed = 40 / 0.88 = 45.5 MWh of surplus solar.
With usable fraction = DoD 0.9 × SoH 0.95 = 0.855, installed capacity = 40 / 0.855 = 46.8 MWh nameplate.
At an indicative ₹4 crore per MWh installed (Indian BESS tender prices have fallen sharply and vary with tender and VGF - verify current numbers), capex ≈ 46.8 × ₹4 crore ≈ ₹187 crore.
Daily cycling for 330 days gives 40 × 330 = 13,200 MWh throughput/year; a ₹3/kWh arbitrage spread yields ≈ ₹3.96 crore/year gross - showing why storage still needs capacity payments or VGF in India.

Aspect	Conventional grid	Smart grid
Power/information flow	One-way, radial	Two-way, reconfigurable
Metering	Monthly manual read	Interval AMI, remote, prepaid
Fault handling	Customer call, crew patrol	FLISR, self-healing
Generation	Central, dispatchable	Central + distributed, variable
Consumer	Passive	Prosumer, demand response
Control	Local, slow SCADA	Wide-area, PMU-based, predictive

Feature	Synchronous generator	Inverter-based RE (grid-following)
Inertia	Inherent (H ≈ 2-6 s)	None unless emulated
Fault current	5-6 pu	1.1-1.5 pu, current-limited
Voltage control	AVR + field current	P-Q curve of converter, limited by kVA
Islanded operation	Possible	Not possible (needs grid-forming)
Response speed	Electromechanical, ~seconds	Power-electronic, milliseconds

Where it is used

System operation: Grid-India NLDC/RLDCs and SLDCs - RE forecasting cells, reserve and ancillary-service scheduling, RE Management Centres (REMCs) at Rajasthan, Gujarat, Tamil Nadu, Karnataka, Andhra Pradesh, Maharashtra and Madhya Pradesh.
Transmission: POWERGRID and state transcos - Green Energy Corridors, pooling substations, STATCOM and reactor projects, HVDC links evacuating Ladakh/Rajasthan RE; roles in system studies (PSS/E, PSCAD, ETAP, DIgSILENT PowerFactory).
Distribution utilities: Tata Power-DDL, BSES, AVVNL, TSSPDCL and others - AMI rollout under RDSS, ADMS/SCADA, rooftop net metering approvals, DT-level loss and theft analytics.
Developers and OEMs: Adani Green, ReNew, Tata Power Renewables, NTPC Green, Suzlon, Vestas, Siemens Gamesa, Hitachi Energy, Sungrow, Tata Power Solar - grid-compliance studies, SCADA and plant controllers (PPC), LVRT type tests.
Storage and EV: BESS tenders by SECI, NTPC and NHPC; PSH projects by NHPC, THDC, Greenko; EV charging management and V2G pilots.
Job roles: grid integration engineer, power system studies engineer, RE forecasting analyst, SCADA/automation engineer, smart-metering project engineer, regulatory/market analyst, O&M engineer at RE plants.

Interview questions

Q: Why does high inverter-based renewable penetration make frequency control harder? A: Inverters supply no inherent inertia, so system H falls and RoCoF after a generation loss rises; the frequency nadir is deeper and reached sooner, leaving less time for governors and load shedding.
Q: Difference between grid-following and grid-forming inverters? A: Grid-following locks a PLL to an existing grid voltage and injects current, so it cannot energise an island; grid-forming controls its own voltage magnitude and angle, supports weak grids, gives virtual inertia and can black-start.
Q: What is anti-islanding and why is it mandatory? A: It is the inverter's detection of loss of mains followed by disconnection (typically within 2 s per IEC 62116/IEEE 1547), protecting line workers from a back-fed dead line and preventing out-of-phase reclosing.
Q: What is LVRT and why not simply trip on a dip? A: LVRT requires the plant to stay connected through a voltage dip following a defined voltage-time envelope while injecting reactive current; mass tripping of RE during a fault would turn a local event into a system-wide generation loss.
Q: How does rooftop solar cause a voltage problem on an LV feeder? A: Reverse power flow makes the voltage rise with distance, ΔV ≈ (PR + QX)/V, so the far end can exceed the statutory limit; mitigated by volt-VAr control, reactive absorption, tap changes or network reinforcement.
Q: What is the duck curve and how is India handling it? A: Midday solar depresses net load and the evening ramp becomes steep; India uses hydro and pumped storage, flexible coal at reduced technical minimum, gas, the real-time market and increasingly BESS with a mandated storage component in RE tenders.
Q: What does "must-run" status mean for RE in India? A: RE generators are scheduled and dispatched without economic curtailment; output may be reduced for grid-security reasons, and commercial curtailment by discoms is contested before regulators.
Q: How does distributed generation affect feeder protection? A: It reduces current seen upstream (blinding), can cause sympathetic tripping of healthy feeders, breaks fuse-recloser coordination and makes flow bidirectional - requiring directional, adaptive and communication-assisted schemes.
Q: Difference between a PMU and conventional SCADA measurement? A: A PMU gives GPS time-synchronised magnitude and phase angle at 25-50 samples/s, so angles across the grid can be compared directly; SCADA gives unsynchronised magnitudes every few seconds with no usable angle.
Q: Net metering versus gross metering? A: Net metering settles exported minus imported energy at the retail tariff; gross metering buys all generation at a feed-in tariff and bills consumption separately - net favours self-consumption economics, gross is simpler for the utility to account for.
Q: What is the short-circuit ratio and why does it matter for a wind farm? A: SCR = grid fault MVA at the point of connection ÷ plant MW rating; below about 3 the grid is weak, converter control loops interact with the network and can oscillate, so grid-forming control or synchronous condensers may be needed.
Q: Why is a BESS specified in both MW and MWh? A: MW fixes the power services it can deliver (frequency response, peak shaving) and MWh fixes for how long; their ratio is the duration or C-rate, and the two are sized by different services.
Q: What is sub-synchronous control interaction? A: An instability between a DFIG's converter control and a series-compensated transmission line at sub-synchronous frequencies, which can grow rapidly and damage equipment - detected by EMT studies and damped by control retuning or filters.

Common mistakes

Treating capacity and energy as interchangeable - quoting GW of solar as if it displaces an equal GW of coal, ignoring capacity factor and firmness.
Forgetting that wind power varies as v³, and that above rated wind speed output is flat because of pitch control.
Assuming an inverter contributes fault current like a synchronous machine, and so sizing or setting protection wrongly.
Confusing inertia (instantaneous, physical, J·ω²/2) with primary frequency response (droop, seconds) and with AGC/secondary control (minutes).
Mixing up net metering, gross metering and net billing, and quoting a single national capacity limit when state SERC rules differ.
Sizing a BESS from energy alone and ignoring the power rating, round-trip efficiency, depth of discharge and state of health.
Quoting IEGC clause numbers, ISTS waiver dates or frequency bands from memory - these are amended often and should be checked against the current notification.
Calling any SCADA-equipped substation a "smart grid" - without two-way data, DER visibility and automated control it is only automation.